Bipolar battery plate

ABSTRACT

A liquid-impermeable plate (10) having through-plate conductivity with essentially zero resistance comprises an insulator sheet (12) having a series of spaced perforations (14) each of which contains a metal element (16) sealingly received into the perforation (14). A low-cost plate can readily be manufactured by punching a thermoplastic sheet (40) such as polypropylene with a punching tool (52), filling the apertures with led spheres (63) having a diameter smaller than the holes (50) but larger than the thickness of the sheet, sweeping excess spheres (62) off the sheet with a doctor blade (60) and then pressing a heated platen (74) onto the sheet to swage the spheres into a cylindrical shape and melt the surrounding resin to form a liquid-impermeable collar (4) sealing the metal into the sheet.

ORIGIN OF THE INVENTION

The invention described herein was made in performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 83-568 (72 Stat435; 42 USC 2457).

This is a division of application Ser. No. 346,414, filed Feb. 18, 1982,now U.S. Pat. No. 4,542,082.

TECHNICAL FIELD

The present invention relates to storage or secondary batteries, morespecifically, the present invention relates to an improved bipolar platefor use in lead-acid batteries.

BACKGROUND ART

The largest single application of lead-acid storage batteries is for thestarting, lighting and ignition of automobiles, trucks and buses. Thesebatteries are charged automatically from a generator driven by theengine while it is running and they supply power for the lights whilethe engine is shut down and for ignition and cranking when the engine isstarted. Lead-acid storage batteries are widely used in aircraft andboats with virtually unlimited applications also existing in non-motivesituations.

Lead-acid batteries are formed from a series of lead-acid cells. Alead-acid cell consists essentially of positive plates containingpositive active materials such as lead dioxide and negative platescontaining negative active material such as sponge lead immersed in anelectrolyte solution, typically dilute sulfuric acid. The respectivepositive or negative plates are connected in parallel. The power orcurrent of the cell is determined by the number and size of the plates.The open circuit potential developed between each positive and negativeplate is about 2 volts. Since the plates are connected in parallel, thecombined potential for each cell will also be about 2 volts regardlessof the number of plates utilized in the cell. One or more cells are thenconnected in series to provide a battery of desired voltage. Common lowvoltage batteries of 6 volts have 3 serially connected cells, 12 voltbatteries include 6 serially connected cells and 24 volt batteriescontain 12 serially connected cells.

The positive and negative plates are typically oriented vertically in ahorizontal stacked relationship. As a result of this verticalorientation, electrolyte stratification commonly occurs vertically alongthe plate surfaces. This results in decreased battery performance. Someattempts have been made to prevent electrolyte stratification such as bystirring electrolyte by means of various mixing systems. These mixingsystems are not only cumbersome but are expensive and subject to failureduring the life of a particular battery.

Another problem with lead-acid batteries is their limited lifetime dueto shedding of the active materials from the positive and negativeplates. Pasted plate lead-acid batteries are by far the most common typeof lead-acid battery. Typically, a paste of lead oxide is applied to thesurface of the positive and negative grids. On application of electricpotential, the lead oxide paste on the positive grid is oxidized to leaddioxide and the lead oxide of the negative grid is reduced to spongelead. During operation these electrode materials shed and flake and falldown between the vertically oriented plates and accumulate at the bottomof the battery. After a period of operation sufficient flakes accumulateto short circuit the grids resulting in a dead battery cell andshortened battery life.

Lead-acid batteries are inherently heavy due to use of the heavy metallead in constructing the plates. Modern attempts to produce lightweightlead-acid batteries, especially in the aircraft, electric car andvehicle fields, have placed their emphasis on producing thinner platesfrom lighter weight materials used in place of and in combination withlead. The thinner plates allow the use of more plates for a givenvolume, thus increasing the power density. Some of these attempts haveincluded battery structures in which the plates are stacked inhorizontal configurations. Higher voltages are provided in a bipolarbattery including bipolar plates capable of through-plate conduction toserially connect electrodes or cells. The horizontal orientation of thegrids prevents the accumulation of flake lead compounds at the batterybottom. Downward movement can be blocked by use of glass mat to containthe electrolyte. Also, stratification of electrolyte is confined andcontained within the acid resistant glass mats by capillary action.

The bipolar plates must be impervious to electrolyte and be electricallyconductive so that electrical current is conducted perpendicularlythere-through to provide a serial connection. The bipolar plates alsopreferably provide a continuous surface to prevent sluffing off ofactive materials from the grids.

Most batteries utilizing bipolar plates have used metallic substratessuch as lead or lead alloys. The use of lead alloys, such as antimony,gives strength to the substrate but causes increased corrosion andgassing. In addition to the problems of forming a liquid tight sealbetween the metallic substrate and adjacent nonconductive case (frame)materials, substrate corrosion, weight and strength factors have alsobeen unacceptable. Furthermore, any attempt to reduce weight has lead toincreased problems of strength and corrosion. Accordingly, a differentapproach must be used if acceptable weight and life are to besimultaneously achieved.

Alternate approaches have included plates formed by dispersingconductive particles or filaments such as carbon, graphite or metal in aresin binder such as polystyrene incorporating therein metal or graphitepowder (U.S. Pat. No. 3,202,545), a plastic frame of polyvinyl chloridewith openings carrying a battery active paste mixed with nonconductivefibers and short noncontacting lead fibers for strengthening thesubstrate (U.S. Pat. No. 3,466,193) a biplate having a layer of zinc anda polyisobutylene mixed with acetylene black and graphite particles forconductivity of the plate (U.S. Pat. No. 3,565,694), a substrate for abipolar plate including polymeric material and vermicular expandedgraphite (U.S. Pat. No. 3,573,122), a rigid polymer plastic frame havinga grid entirely of lead filled with battery paste (U.S. Pat. No.3,738,871), a plastic thin substrate having lead stripes on oppositefaces, the lead stripes being interconnected through an opening in thesubstrate, and retained by plastic retention strips (U.S. Pat. No.3,819,412) and a biplate having a substrate of thermoplastic materialfilled with finely divided vitreous carbon and a layer of lead antimonyfoil bonded to the substrate for adhering active materials (U.S. Pat.No. 4,098,967).

Some more recent examples of batteries containing bipolar plates areU.S. Pat. No. 4,275,130 in which the biplate construction comprised athin composite of randomly oriented conductive graphite, carbon or metalfibers imbedded in a resin matrix with stripes of lead plated onopposite surfaces thereof. My copending application Ser. No. 279,841filed July 2, 1981 entitled BIPOLAR SEPARATE CELL BATTERY FOR HIGH ORLOW VOLTAGE includes a biplate formed of a thin sheet of titanium iscovered with a layer of epoxy resin containing graphite powder.

The resistance of such plates is always higher than predicted or desireddue to the conduction path being formed of point to point contact of thedispersed conductive particles which are surrounded by highly insulativeresin materials. The through-plate serial conductivity is limited andsince the voltage of the cell is increased by including more bipolarplates; this increases the resistance of each cell and of the battery.

SUMMARY OF THE INVENTION

It has now been discovered in accordance with the present invention thatan electrolyte-impermeable bipolar plate can be provided havingessentially no grid resistance and allowing no sluffing of activematerials. The bipolar plate of the invention has a conductivitysubstantially higher than the lightweight, thin composite bipolar platesdiscussed above.

The bipolar plate of the invention is formed of a continuous sheet ofresinous material containing a plurality of spaced conductors extendingfrom a first surface to the second surface thereof. The conductors aresealingly received in the sheet of resin such that no liquid passesbetween the resin enveloping the end of the conductor facing eachsurface thereof. The bipolar plate of the invention can readily bemanufactured in a very low cost, repetitive manner to provide alightweight, inexpensive biplate.

One very simple efficient manner for producing the bipolar plate of theinvention is to provide a series of spaced apertures in a thermo-plasticsheet that are slightly larger in diameter than metal pellets. Thepellets are preferably sized of a diameter slightly smaller than thediameter of the holes but somewhat larger than the thickness of thesheet. The sheet is then passed through a chamber where it is coveredwith lead pellets. The pellets enter the apertures. The sheet is thenmoved upwardly in an amount equal to the excess diameter of pellets. Theexcess pellets are swept from the top of the sheet and the sheet is thenrolled or pressed with a heated platen whereby the plastic melts and thelead pellets are partially flattened and expanded forcing the lead intothe plastic. When the plastic cools it forms a seal completely aroundeach lead pellet which is now in the form of a cylinder having oppositeends exposed coincident to the opposite surfaces of the sheet. Theprocess is capable of very high production rates to form low costbipolar plates in a simple and efficient manner.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view in elevation of a bipolar plate in accordancewith the invention;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is an enlarged view of a portion of the bipolar plate;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 3;

FIG. 5 is a sectional view of a cell for a bipolar battery; and

FIG. 6 is a schematic view of a continuous system for manufacturingbipolar plates according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 and 2 a bipolar plate 10 in accordance with theinvention comprises an insulator sheet 12 in which is formed a patternof spaced perforations 14. Each perforation 14 containing a conductivemetal element 16 having a face 18 facing the front surface 20 of theplate 10 and a face 22 facing the opposite surface 24 of the plate 10.The elements 16 are sealingly received in the perforations 14 such thatthe sheet 12 is impervious to liquid electrolyte. The plate ispreferably thin to reduce mass and volume of the battery. The thicknessof the sheet is generally from 5 mils to 100 mils, preferably from 10 to30 mils. The metal elements generally occupy about 10 to 40% by volumeof the sheet. The elements can have any geometrical configuration. Theypreferably are cylindrical in shape but may have a rectangular or otherpolygonal cross-section. The preferred diameter size of the elements isin the range of 30 to 60 mils providing a preferred center-to-centerspacing of 150 to 250 mils.

The insulator sheet can be formed of any liquid impermeable electricalinsulator but is preferably formed of a synthetic organic resin which isinert to the electrochemical environment of the cell. Flexible resinsare more suitable to avoid brittle failure. Thermoplastic resins arerequired in the mass production technique of the present invention sincethey more readily form fluid-tight seals with the metal elements.Polyolefins, particularly the aliphatic polyalkenes such as polyethyleneor polypropylene, are thermoplastic, resilient and inert to theelectrolyte. Polypropylene and polyethylene also melt at a temperaturebelow that of the low melting metals such as lead which is necessary toavoid perforation of the metal element. Polypropylene is the material ofchoice since it is a tough, resilient, thermoplastic, melting below themelting point of lead and has excellent resistance to theelectrochemical environment of the lead-acid battery, possibly exceedingthat of epoxy resins.

The metal elements can be formed of any good conductor inert to theelectrochemical environment of the cell. Lead is most preferred due tothe inert character of this material, its ready availability andmoderate cost. The elements can be formed into the sheet by castingtechniques. The preferred manufacturing process of the inventionproceeds by placing the elements into preformed apertures and sealingthe elements into thermoplastic sheets.

Though the optimum shape for the element is a short cylinder, it isdifficult to implant this shape and reliably seal it into a preformedaperture. A spherical element is not an optimum shape. However, spherescan readily be inserted into the apertures and by a combination of heatand pressure, the sphere is deformed by swaging to a shape approximatinga cylinder and the surrounding thermoplastic resin is melted, stretchedand compressed to provide a liquid impermeable sheath around theelement.

In order to practice this technique, the aperture should be slightlylarger than the diameter of the sphere, generally providing a clearanceof at least 5 mils to 20 mils. The spheres should have a diameter 10 to30 mils larger than the thickness of the sheet and a volume exceedingthat of the aperture so that the spheres can be swaged into cylindricalshape while forming a seal with the surrounding resin.

Referring now to FIGS. 3 and 4, as a result of the application of aheated platen to the sheet 10 containing the spheres, the top surface ofspheres will be swaged or mashed into a flat disc 30, the surroundingresin will melt and flow onto the peripheral curved edge 32 to form asealing ring 34. The element retains its spherical shape along the sideportions 36 within the sheet.

Referring now to FIG. 6, a continuous, low-cost process for rapidlyproducing lightweight bipolar plates with essentially zero electricalresistance comprising the steps of perforating a thermoplastic sheet,filling the perforations with metal spheres having a diameter smallerthan the perforations but larger than the thickness of the sheet,raising the sheet to sweep off excess spheres and heating and pressingthe sheet with a heated platen or roller to swage the spheres into thesheet. The sheet can be rolled or cut into lengths for use as bipolarplates.

The thermoplastic sheet 40 is passed from supply roll 42 past theperforation station 44, filling station 46, and pressing station 48before being cut into lengths or rerolled. While in the perforationstation 44 of the equipment train, a series of pilot holes 50 are formedin the sheet 40 by vertically passing punching tool 52 containing aplurality of punch rods 54 through the sheet 40.

The perforated sheet then enters chamber 56 of the filling station.Spheres are fed onto the surface 58 of the sheet 40 from storagecontainer 64. Retractable rollers 66 are then actuated to raise thesheet so that the top of the spheres are not protruding above the topsurface 58 of the sheet. The sheet then passes under doctor blade 60which sweeps the excess spheres 62 off the sheet and into basin 70.

Support rollers 66 are then retracted and the sheet-sphere assembly ispassed between stationary support 72 and heated platen 74. The heatedplaten is moved downwardly to apply heat and pressure to the sheet whichresults in flattening of the ends of the spheres and heating, softeningand sealing of the surrounding resin. The sheet is optionally cooled bypassage through a cooling station 80. Blade 76 is then actuated to formindividual bipolar plates 78.

In a particular embodiment a 65-mil thick polyethylene sheet was drilledto provide apertures having a diameter of about 100 mils. Lead pelletshaving a diameter of about 85-90 mils were placed in each apertures. Thesheet was heated and pressed to seat and seal the pellets in the sheet.Twenty percent of the volume of the final sheet is lead. The weight of abipolar plate, the size and shape of a grid from a Globe-Union EV 2-13battery would be 36.6 grams. This compares to 120.9 grams for the EV2-13 grid and the resistance of the bipolar plate of the invention willbe lower by a factor of several million.

Though the bipolar plate has been exemplified with lead conductors forthe lead-acid battery, it also could be utilized with nickel, iron, zincor cadmium pellets in a polyolefin such as polypropylene fornickel-alkaline batteries.

The bipolar plate of the invention is liquid impervious, zeroresistance, through plate conductor having application in any stackedelectrochemical cell in which it is desired to provide conduction to anadjacent electrode or an adjacent cell. The plate can be used inbatteries, electrolysis cells, fuel cells, electrophoresis cells, etc.The plate can be used in cells with vertically or horizontally disposedcells. The preferred cell configuration is horizontal since horizontaldisposition of a cell prevents electrolyte stratification and thecontinuous, flat surface of the bipolar plate of the invention willprevent shedding of active electrode material, the most prominentfailure mode for lead-acid cells.

A particular, efficient, horizontal battery configuration is disclosedin my copending application, Ser. No. 279,841, filed July 2, 1981,entitled BIPOLAR SEPARATE CELL BATTERY FOR HIGH OR LOW VOLTAGE, thedisclosure of which is expressly incorporated herein by reference. Inthat application, bipolar plate groupings are placed between monopolarplates to increase available potential voltage. The conductive plate ofthe invention can be utilized as a substrate to form either the bipolarplate or monopolar plate of such a battery. A monopolar plate willdiffer in having the same polarity material provided on each surfacethereof, and means to provide laterial conduction to provide forparallel connection of cell groupings.

Referring now to FIG. 5 a biplate grouping 90 can be assembledsurrounding a biplate 92 of the invention by supporting a layer 94 ofpositive active lead dioxide material on a first glass scrim sheet 96and a layer 98 of negative active sponge lead on a second sheet 100 ofglass scrim. These sheets 96, 100 are then placed against the plate 92with the active layers 94, 98 in contact with the surfaces of the plate82 and with the metal elements 102. The scrim sheets are in turn facedwith a porous, fibrous mat 104 suitably formed from glass fibers. Theporous mat is capable of releasing any gases formed during operation ofthe cell and holds the electrolyte. The sheets of scrim fabric may bebonded to the mats 104. By bonding an opposite polarity scrim sheet 106,108 to each mat 104, a bipolar grouping can be assembled by alternatinglayers of plates 92 with bipolar porous mat assemblies 110.

The bipolar groupings can be interspersed with monopolar platesconnected by bus bars to battery terminals. Alternately, the electrodematerials can be plated directly onto the through conducted substrateplate of the invention. For example, sponge lead can be coated onto onesurface and lead dioxide can be coated directly onto the other surfaceor indirectly onto lead stripes coated onto the opposite surface.Bipolar groupings are formed simply by interspersing a porouselectrolyte-separator plate between the active material coated bipolarplate. The active materials can be applied as pastes and cured on thescrim or plate according to state of the art procedures. The activematerials can also be formed in situ according to the state of the artby applying lead to each surface and then placing the electrodematerials in electrolyte and connecting them to a source of potential.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

I claim:
 1. A method of forming a bipolar plate for a battery comprisingthe steps of:disposing metal pellets in each aperture of a perforatedthermoplastic sheet to form an assembly; heating and pressing theassembly to seal the pellets into the apertures with first and secondsurfaces exposed in the opposite faces of the sheet.
 2. A methodaccording to claim 1 in which the resin has a melting point below thatof the metal.
 3. A method according to claim 2 in which the resin is apolyolefin.
 4. A method according to claim 3 in which the polyolefin ispolypropylene.
 5. A method according to claim 3 in which the metal islead.
 6. A method according to claim 1 in which the sheet has athickness from 5 to 100 mils.
 7. A method according to claim 6 in whichthe apertures are spaced in the sheet in a uniform pattern with thedistance from center to center being from 150 to 250 mils.
 8. A methodaccording to claim 1 in which the pellets are spheres and the apertureshave a diameter slightly larger than the spheres and the sheet has athickness slightly less than said diameter.
 9. A method according toclaim 8 in which the pellets are disposed in the apertures by moving thesheet through a chamber and rolling pellets across the top surface ofthe sheet and removing excess pellets from the top surface.
 10. A methodaccording to claim 9 in which the pellets are sealed into the aperturesby placing the sheet on a rigid surface and pressing a heated platenagainst the top surface to swage the pellets into the softenedthermoplastic sheet.
 11. A method according to claim 1 in which thethermoplastic sheet is formed of an organic resin insulator.
 12. Amethod according to claim 10 in which the pellets are swaged to formflat first and second surfaces coincident with the flat upper and lowersurfaces of the sheet.